1. Field of the Invention
The present invention relates to a deep hole cutter such as a drill for application to a gun drill system.
2. Description of the Related Art
Gun drill systems, BTA systems, ejector systems, etc. are known as deep hole cutting systems. In general, gun drill systems that are simple in structure are used to cut deep holes that are relatively small in diameter.
As shown in FIG. 12, a gun drill system includes a gun drill 30 consisting of a tubular tool shank 31 and a drill head 32. The tool shank 31 has an outer axial discharge groove 34 V-shaped in radial section. The drill head 32 is fixed to the front end of the tool shank 31. The interior of the tubular shank 31 serves as a supply passage 33. While the gun drill 30 is cutting a deep hole H in a work W,it produces shavings S in the hole H. In the meantime, a high pressure coolant C is supplied through the supply passage 33 and discharged from the front end of the drill head 32 so as to expel the shavings S through the discharge groove 34. Even if the gun drill 30 is small in diameter, the discharge groove 34 can be large in radial section so that the shavings S are relatively easy to discharge.
FIG. 13 shows a generally known gun drill 30 of the foregoing type, which consists of a tool shank 31 and a drill head 32. The tool shank 31 consists of a cylindrical driver 31a and a tubular shaft 31b. The cylindrical driver 31a is held by a chuck (not shown) or the like to be rotated. The tubular shaft 31b is made of pipe material having a relatively thin wall, and its root is fixed in the cylindrical driver 31a. A front part of the tubular shaft 31b has an outer axial discharge groove 34a formed by die molding and V-shaped in radial section. The drill head 32 is made of steel, which is ground so that an outer discharge groove 34b similar to the discharge groove 34a is formed along its length. A carbide tip 35 is brazed to the front end of the drill head 32. The cutting edge of the carbide tip 35 extends along the front end of the trailing side of the V-shaped groove 34b. The drill head 32 has two axial supply bores 33b formed through it and communicating with the supply passage 33a of the tool shank 31. The drill head 32 also has two outlet ports 36 formed in its front end and each communicating with one of the supply bores 33b. The drill head 32 may be replaced with another drill head the whole of which is made of tool steel, and the front end of which is ground to be formed with a cutting edge. The drill head 32 may be replaced with still another drill head to which a carbide tip is screwed. The front end of the tubular shaft 31b is notched or recessed in the form of the letter V. The rear end 32a of the drill head 32 protrudes in the form of the letter V. These ends are engaged and brazed together so that the tool shank 31 and the drill head 32 are fixed together.
If the carbide tip 35 is worn or broken, it is necessary to replace the whole drill 30 inclusive of the tool shank 31, so that the costs of the drill are high. For rearrangement, it takes a long time to replace the long drill 30, so that the working efficiency is low. When the carbide tip 35 is worn, it is not easy, but costly to regrind the tip. In order to ream a hole drilled by the gun drill 30, it is necessary to provide a reamer including a reamer head, which is fixed to the front end of a tool shank 31. This makes the costs even higher.
In view of the foregoing, the applicant proposed, in Japanese Patent Application No. 2002-295789, a deep hole cutter consisting of a tubular tool shank and a detachable cutting head. The interior of the tubular shank serves as a coolant supply passage. The tool shank has an outer axial discharge groove V-shaped in radial section. The cutting head is screwed detachably into the front end of the tool shank.
FIG. 9 shows a deep hole cutter according to the applicant's proposal, which consists of a tool shank 21 and a cutting head 22. The tool shank 21 consists of a cylindrical driver 21a, a tubular shaft 21b made of pipe material and a connector 21c, which consists of a rear part and a tubular front part. The root of the tubular shaft 21b is fixed in the cylindrical driver 21a. The rear end of the connector 21c is fixed to the front end of the tubular shaft 21b. The interior the cylindrical driver 21a and tubular shaft 21b serves as a coolant supply passage 23a. The rear part of the connector 21c has two axial bores formed through it, one of which is shown, and which communicate with the supply passage 23a. The tool shank 21 has an outer axial discharge groove 24a V-shaped in radial section. The discharge groove 24a extends along a front part of the tubular shaft 21b and the whole length of the connector 21c. 
The tubular front part of the connector 21c has a female thread 27a formed at its bottom. The discharge groove 24a extends across the female thread 27a. 
As shown in FIGS. 9 and 10(A)–10(D), the cutting head 22 has a male thread 27b formed at its root, two axial supply bores 23b formed through it, an outer discharge groove 24b formed along its length, a carbide tip 25 fixed to its front end and two outlet ports 26 formed in its front end. The male thread 27b engages with the female thread 27a of the connector 21c. Each supply bore 23b communicates with one of the supply bores of the connector 21c when the cutting head 22 is connected with the connector 21c. The discharge groove 24b is V-shaped in radial section and extends across the male thread 27b. The discharge groove 24b is aligned with the discharge groove 24a of the tool shank 21 when the cutting head 22 is connected with the connector 21c. The cutting edge of the carbide tip 25 extends along the front end of the trailing side of the discharge groove 24b. Each outlet port 26 is the front end of one of the communicating bores 23b. 
If the carbide tip 25 is worn or broken, it is possible to replace only the cutting head 22 so as to keep using the tool shank 21. This greatly reduces the costs of the hole cutter in comparison with the hole cutter of which the cutting head is fixed to the tool shank. For rearrangement, it is possible to replace only the cutting head 22 by unscrewing it from the connector 21c and screwing a new cutting head into the connector 21c. The replacement can be done simply in a short time so that the production efficiency can be improved. When the carbide tip 25 is worn, it is possible to regrind or replace the tip easily by detaching only the cutting head 22. If it is necessary to switch from drilling to reaming or another cutting operation, only an appropriate cutting head can be provided for the operation. This reduces the part/component costs of the hole cutter and makes it possible to replace the cutting head 22 easily in a short time.
Drills each having a plurality of cutting edges are used widely as oil hole tools for cutting holes that are relatively large in diameter. The cutting edges of each of the drills are formed on its diametrically opposite sides at its front end. The cutting edge or edges on one of the sides face in one direction, and that or those on the other side face in the opposite direction. If a gun drill system includes a cutting head having such cutting edges, it needs to have outer axial discharge grooves for efficiently discharging shavings while it is drilling a hole. The discharge grooves are V-shaped in radial section and each extend on one of diametrically opposite sides of the cutting head and tool shank.
The deep hole cutter shown in FIGS. 9 and 10 might have outer axial discharge grooves formed on diametrically opposite sides of its tool shank 21 and cutting head 22. These discharge grooves would, however, reduce the threads 27a and 27b in area, greatly lowering the strength of the connection between the tool shank 21 and cutting head 22, so that the connection could not withstand cutting loads.
FIGS. 11(A) and 11(B) show an assumed structure of a deep hole cutter for application to a gun drill system. This hole cutter consists of a cutting head 22P and a tool shank (not shown). The cutting head 22P can be screwed detachably into the tool shank and has three cutting edges. FIG. 11(A) is a front end view of the cutting head 22P. FIG. 11(B) is a radial section of the root of the cutting head 22P. The cutting head 22P has a large outer discharge groove 24L and a small outer discharge groove 24S that are V-shaped in radial section. The discharge grooves 24L and 24S are formed on diametrically opposite sides of the cutting head 22P and extend along its whole length. The cutting head 22P is fitted with a central tip 25a, an intermediate tip 25b and a peripheral tip 25c on its front end. The cutting edges of the central and peripheral tips 25a and 25c extend along the front end of the trailing side of the large discharge groove 24L. The cutting edge of the intermediate tip 25b extends along the front end of the trailing side of the small discharge groove 24S. The cutting head 22P also has two coolant supply bores 23 each formed axially through one of its parts 28a and 28b that are nearly fan-shaped in radial section between the discharge grooves 24L and 24S. The cutting head 22P further has two outlet ports 26 formed in its front end, each of which is the front end of one of the supply bores 23. Each of the fan-shaped parts 28a and 28b is fitted with a guide pad 29 on its peripheral surface.
As shown in FIG. 11(B), the cutting head 22P has a male thread 27b formed at its root for engagement with the female thread of the tool shank (not shown). The male thread 27b is interrupted by the discharge grooves 24L and 24S and separated into two groups on the fan-shaped parts 28a and 28b. Moreover, the sum of the circumferential lengths of the fan-shaped parts 28a and 28b is only slightly larger than ½ of the whole circumference of the cutting head 22P. This is also the case with the female thread of the tool shank. Accordingly, the screw connection between the cutting head 22P and the tool shank is very weak in strength, so that cutting loads are liable to break, bend, twist or otherwise deform the connection.